Method for welding parts made of thermoplastic material

ABSTRACT

A method for welding at least two parts including a thermoplastic material and having respective surfaces to be welded, including: inserting an insert between the surfaces to be welded of the two parts; generating heat via the insert; wherein the insert moves in relation to the parts to be welded in a welding direction. Also, an installation adapted for implementation of the method.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.17/261,008, filed on Jan. 16, 2021, which is a U.S. national stage ofInternational Application No. PCT/FR2019/051775, filed on Jul. 16, 2019,which claims the benefit of French Application No. 1856537, filed onJul. 16, 2018, French Application No. 1905222, filed on May 17, 2019,and French Application No. 1905223, filed on May 17, 2019. The entirecontents of each of U.S. application Ser. No. 17/261,008, InternationalApplication No. PCT/FR2019/051775, French Application No. 1856537,French Application No. 1905222, and French Application No. 1905223 arehereby incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention concerns a welding method, in particular aninduction welding method and an installation for implementing thismethod.

TECHNICAL BACKGROUND

Composite materials comprising reinforcing fibres e.g., carbon fibresand/or glass fibres dispersed in a thermoplastic polymer matrix havenumerous possible uses and in particular for the manufacture of aircraftfuselages in the field of aeronautics.

The dispersion of fibres in a thermoplastic polymer matrix impartsparticular properties to rigid composite parts, in particular in termsof crack resistance, fatigue strength, recyclability.

Parts in composite materials are typically composed of several plies(layers) that are superimposed and laminated together, the fibres ineach ply having a main direction that most often differs from the maindirection of the fibres of adjacent layers. These plies (or layers)layers are also called “semi-finished products”, which can be obtainedfor example by impregnating continuous, generally unidirectional fibreswith resin. There are different production methods whereby the resin canbe melted, or dissolved in a solvent, or is in powder form, in afluidized bed, or dispersed in an aqueous suspension. The impregnatedfibres are optionally stripped of the solvent or water then heatedbefore melting the retained resin and forming the semi-finished product.It is also possible to cause the thermoplastic resin to enter thereinforcing fibres by closely mixing (co-mixing) the reinforcing fibreswith thermoplastic fibres that are melted to form the resin surroundingthe reinforcing fibres. The semi-finished products can also be obtainedby impregnating a fibrous fabric or assembly of stitch-bondedunidirectional fibres (Non-Crimp Fabrics—NCF) with a polymer powderwhich, once melted, will form the matrix of the composite. Asemi-finished product is characterized by homogeneous distribution ofresin (then called a matrix) around the reinforcing fibres.

For some applications, it may be necessary to associate severalcomposite parts e.g., for an aircraft fuselage, skin panel, stiffenersand frames.

These composite parts can be assembled by welding, in particular bytechnology of induction type. This process uses an inductor emitting amagnetic field. This magnetic field causes a rise in temperature ofinduction-sensitive materials, up to a temperature suitable for weldingthermoplastic polymers.

At the current time, the induction welding of composite parts involveseither direct heating of the composite materials to be assembled, orheating an insert (or susceptor) reacting to the magnetic field andintrusive (generally in metal material) previously deposited at theinterface to be assembled.

However, a method based on direct heating of the carbon fibres of thecomposite parts to be assembled has the following disadvantages:

-   -   implementation thereof requires that the carbon fibres be        continuous and have orientations or intermingling which promote        the creation of current loops in the material;    -   this method does not generally allow localization of the heat at        the interface alone; this often leads to heating the whole        composite part which, if not corrected by suitable coolers, can        translate as risks of decompaction possibly causing delamination        of the layers of the composite part; the heat produced can also        affect zones bordering the weld line;    -   since these zones are not subjected to compacting pressures,        they may suffer the same negative effects as those cited above;    -   it is generally necessary to add an induction-sensitive        material, such as a thermoplastic film containing fillers or        conductive or ferro-electrical structures, at the weld        interface; the addition of such films makes the certification of        welded parts for aeronautic applications very difficult to        obtain;    -   it is not possible to use the induction welding technique when        the reinforcing fibres of the composite material are not        conductive or ferromagnetic (e.g., glass fibres, aramid fibres .        . . );    -   this method is sensitive to the type, configuration (i.e.,        lay-up) and thickness of the parts to be welded;    -   this method, applied to the welding of carbon composite parts,        does not allow guaranteed welding parameters in the start and        end zones of the weld and therefore the homogeneity of the weld        joint using this dynamic technology.

To overcome these problems, different strategies have been put forward.

For example, document WO 2013/110270 describes an induction weldingmethod wherein a cooling unit follows the inductor so that the surfaceof the composite part facing the inductor is cooled and does not melt.

Document EP 1849581 concerns an induction welding device to secure amoulded plastic part onto the surround of a tubular part composed of atleast one metal layer and a layer of thermoplastic resin, the devicecomprising an element having extensive magnetic permeability to channelthe magnetic field lines.

As indicated above, another solution frequently employed is to insert asusceptor composed of a material that is more induction-sensitive thancarbon e.g., a metal mesh at the interface of the parts to be welded. Byadapting the intensity of the emitted magnetic field, it is possible tolocalize heating at the susceptor and hence at the interface of theparts to be welded.

For example, document EP 2907651 describes an induction welding methodwhereby an assembly formed of two parts to be welded and a fieldabsorber (or susceptor) positioned at the interface of the parts issubjected to a magnetic field by an inductor at a particular incidence.

Document EP 20150393 describes an induction welding method wherein twoparts are placed in a mould for coupling therein, at least one contactsurface between the parts comprising heat-activated coupling means andan induction-sensitive component.

Document WO 2012/158293 describes an induction welding method wherein asusceptor is placed between two composite parts and a magnetic fieldparallel to the susceptor is generated.

Document EP 0720906 describes a thermoplastic welding method wherein asusceptor is placed at the interface of the two resin parts to bewelded.

Document U.S. Pat. No. 5,753,058 relates to thermoplastic weldingapparatus for welding composite parts comprising a conductive susceptorat the interface to be welded.

Document U.S. Pat. No. 5,902,935 relates to a method for evaluating theintegrity and strength of a thermoplastic weld in which a susceptor isincorporated.

Document U.S. Pat. No. 6,323,468 describes induction welding apparatusfor assembling two components generating a magnetic field to causeheating of a susceptor placed between the two components to be welded.

Document WO 2008/087194 describes an induction method to weld athermoplastic material to a composite material comprising a heat-settingmatrix reinforced with fibres, wherein preferably a conductive materialis positioned at the interface of the materials to be welded and isheated by induction.

Document U.S. Pat. No. 4,978,825 describes an induction method to weldan assembly comprising two parts between which a susceptor is placedwhich is heated by induction, the inductor being incorporated inside apressure roller.

Document WO 2015/140270 concerns a thermoplastic welding method to weldtwo parts in thermoplastic composite material. Metal inserts are placedbetween the two surfaces of the parts to be welded so that induced,heat-producing currents are generated in these inserts, the assembly tobe welded being enclosed in a sealed chamber in which a partial vacuumis applied.

However, the use of a susceptor may result in a non-homogeneous weld andhas the disadvantage of inserting a non-desirable third body in the weldassembly. The presence of a third body at the interface of the partswhich have been welded may in particular prevent or restrict the use ofthe welded parts in the field of aeronautics.

Document FR 2488828 concerns a method for welding sheets ofthermoplastic material corresponding to flexible sheets which are likelyto ripple and form creases. This method consists in particular ofplacing the two sheets to be welded so that their adjacent edgesoverlap, moving a hot wedge between the overlapping edges, providing acertain amount of thermoplastic material and pressing the overlappingedges allowing them to cool. Document FR 2488828 does not describe thewelding of rigid parts.

There is therefore a real need for providing an efficient, rapid methodhaving good performance for assembling parts in thermoplastic materials,in particular rigid parts in thermoplastic materials, and which avoidsthe above-mentioned disadvantages.

There is also a true need to provide an assembly method, using localizedheating at the interface of parts in thermoplastic materials, thatperforms well, is efficient, rapid and dynamic.

There is a true a need to provide a method allowing the assembly ofparts by heating. In particular, there is a true need to provide anefficient method allowing the assembly of parts by heating withoutfiller material, without deformation, without delamination and withoutdecompaction of the parts to be welded.

SUMMARY OF THE INVENTION

The invention first relates to a method for welding at least two parts,in particular two rigid parts, comprising a thermoplastic material andhaving respective surfaces to be welded comprising:

-   -   inserting an insert between the surfaces to be welded of the two        parts, said insert having a thickness of 5 mm or less;    -   providing heat by said insert;        wherein the insert moves in relation to the parts to be welded        throughout welding, in a welding direction.

In some embodiments, the heat is provided by said insert which is heatedvia induction, via resistive effect, via vibration, via friction, viaultrasound, via use of laser, via a stream of hot gas or via conductionfrom an external heat source.

In some embodiments, the insert comprises an induction-sensitivematerial, and the heat of the insert is provided through the generationof a magnetic field by at least one inductor.

In some embodiments, the insert and the inductor move together inrelation to the parts to be welded throughout welding in the weldingdirection.

In some embodiments, the insert comprises an electrically conductivematerial, and the heat of the insert is provided via resistive effect.

In some embodiments, the method further comprises the contacting of thewelding surfaces of the two parts to be welded by applying pressure ontoat least one of the two parts upstream and/or downstream of the positionof the insert in relation to the welding direction.

In some embodiments, the method further comprises a step to cool thefree surfaces of the parts to be welded, in particular by applying aheat-regulating block onto at least one of the two parts behind theposition of the insert in relation to the welding direction, and infront of the pressure-applying element(s) if any.

In some embodiments, the insert is in contact with each of the surfacesto be welded of the two parts.

In some embodiments, the insert is not contact with at least one of thesurfaces to be welded of the two parts.

In some embodiments, the method further comprises the movement, togetherwith movement of the insert, of a spacer element between the surfaces tobe welded of the two parts, the spacer element being positioned ahead ofthe insert in the welding direction.

In some embodiments, at least one of the two parts, preferably bothparts, are in composite material comprising reinforcing fibres in amatrix of the thermoplastic material.

In some embodiments, the reinforcing fibres are carbon fibres and/orglass fibres, or any other type of fibre able to reinforce orfunctionalize a polymer.

In some embodiments, at least one of the two parts, preferably bothparts, essentially consist, or consist, of the thermoplastic material.

In some embodiments, the thermoplastic material is selected from thegroup of polyamides, polyimides in particular polyetherimides,polyaryletherketones in particular polyetherketoneketones andpolyetheretherketones, ethylene polyterephtalates, polyolefins inparticular polypropylene, phenylene polysulfides, polysulfones,chlorinated polymers in particular polyvinyl chloride (PVC) andpolyvinylidene fluoride (PVDF), acrylic or methacrylic polymers, and itis preferably a polyaryletherketone such as polyetherketoneketone orpolyetheretherketone.

In some embodiments, at least one of the two parts, preferably bothparts, are a multilayer structure.

In some embodiments, the layer comprising the surface to be welded of atleast one of the two parts, preferably of both parts:

-   -   comprises a thermoplastic material having a melting point lower        than that of the thermoplastic material of the other layers of        the parts; and/or    -   comprises a thermoplastic material having lesser viscosity than        that of the thermoplastic material of the other layers of the        parts; and/or comprises a volume amount of thermoplastic        material greater than that of the other layers of the parts;        and/or    -   comprises a reinforcing material of strong cross density,        preferably a woven carbon fabric; and/or    -   comprises a unidirectional fibre layer oriented in the welding        direction.

In some embodiments, the method does not comprise a step to addadditional thermoplastic material, in particular at the interface of thesurfaces to be welded.

In some embodiments of the induction welding method, the insertcomprises an induction-sensitive metal material. The insert isoptionally fully or partially coated with a functional coating forexample affording anti-corrosion properties (anti-corrosion material) orfacilitating the sliding of the insert between the two parts (materialfacilitating sliding of the insert between the two parts).

In some embodiments of the induction welding method, the insert consistsof a ferromagnetic material having a Curie temperature Tc, which couldfacilitate control over the method.

In some embodiments, the method further comprises the formation of abead (or meniscus) of thermoplastic material at the end of the weldinterface.

In some embodiments, the parts are parts of an aircraft fuselage.

The invention further relates to an installation for welding at leasttwo parts, in particular two rigid parts, comprising a thermoplasticmaterial and having respective surfaces to be welded, comprising:

-   -   a support to hold the two parts to be welded;    -   an arm having at its end portion a heating insert configured to        be inserted between the surfaces to be welded of the two parts;    -   the insert, of thickness 5 mm or less, being configured to move        in relation to the parts to be welded throughout welding in a        welding direction.

In some embodiments, the installation also comprises a device togenerate the heat of said insert via induction, via resistive effect,via vibration, via friction, via ultrasound, via laser, via a stream ofhot gas or via conduction from an external heat source.

In some embodiments of an induction welding installation, theheat-generating device is at least one inductor, and said insertcomprises an induction-sensitive material.

In some embodiments, the insert and inductor are configured to movetogether in relation to the parts to be welded, throughout welding inthe welding direction.

In some embodiments, the arm carrying the insert at its end portion isattached to the inductor.

In some embodiments of a welding installation via resistive effect, theheat-generating device is a device generating an electrical current.

In some embodiments, the installation further comprises one or morecompacting rollers and/or one or more pressure rollers.

In some embodiments, the installation also comprises at least oneheat-regulating block.

In some embodiments of the induction welding installation, thecompacting rollers and/or pressure rollers are attached to the inductor.

In some embodiments, the compacting roller(s) are configured to bevibrated at an adapted frequency. This vibration is adapted to optimizethe interpenetration phenomena required to obtain high-performancewelding.

In some embodiments, the installation comprises a controlled-temperaturechamber preferably comprising a flexible skirt.

In some embodiments, the insert is a plate of thickness 5 mm or less,preferably of 0.3 to 5 10 mm, more preferably 0.3 to 3 mm, furtherpreferably 0.5 to 1.5 mm.

In some embodiments of the induction welding installation, the insertcomprises an induction-sensitive metal material and is optionally fullyor partially coated with a functional coating for example affordinganti-corrosion properties (anti-corrosion material) or facilitating thegliding of the insert between the two parts (material facilitatinggliding between the two parts).

In some embodiments of the welding installation via resistive effect,the insert comprises an electrically conductive material and isoptionally fully or partially coated with an insulating material.

In some embodiments, the installation further comprises a second armhaving at its end portion a spacer element optionally attached to theinsert.

In some embodiments, the support is configured to be heated.

With the present invention, it is possible to overcome the disadvantagesof the prior art. More particularly, it provides a method that performswell, is efficient and rapid for assembling parts in thermoplasticmaterials and in particular rigid parts in thermoplastic materials. Inparticular, the method of the invention does not require the permanentinsertion of a third body in the welded assembly, whilst allowinglocalized heating preferably at the interface of the parts to be welded.Localized heating in the interface area to be welded allows limiting ofthermal effects in the other plies of the composite part and therebyprevents any deterioration of the quality of the parts to be assembledthrough delamination and/or decompaction. In general, the quality of theelementary parts before assembly has been controlled and validated; itis therefore of high interest to have available a welding technologywhich does not jeopardize the quality of the parts as provided by thepresent invention.

This is achieved through the use of an insert providing heat and whichis in movement in relation to the parts to be welded in the weldingdirection. The heat can be generated by any adapted means, in particularvia induction, via resistive effect, via induction, vibration, viafriction, via ultrasound, via use of laser, via a stream of hot gas orvia conduction from an external heat source; in particular via inductionor resistive effect. For example, the magnetic field created by theinductor produces effects which are concentrated in the insert, inducinglocalized heating at this insert. Similarly, the electrical currentgenerates heat within the insert via resistive effect. The insert movesin relation to the parts to be welded in the welding direction and istherefore not integrated in the final assembly.

Additionally, the invention has one or preferably more of the followingadvantages:

-   -   the method of the invention allows the welding of rigid parts;    -   the method of the invention allows the welding of all types of        thermoplastic materials, including materials not comprising        conductive elements (such as carbon fibres or other fibres, or        conductive fillers);    -   for induction welding, the method can allow the use of reduced        induction power compared with methods based on heating the        carbon contained in the composite parts to be welded;    -   the method can allow better control over welding temperature;    -   the method can allow the welding of large-size parts and/or of        complex geometry such as double bends;    -   the method can allow some tolerance with regard to        complementarity to be heeded by the surfaces to be welded;    -   for induction welding, the method can allow reduced consumption        of energy and materials, since heating is localized at the        surfaces to be welded by material that is more        induction-sensitive than the materials of the parts to be        welded;    -   associated for example with the choice of optimised material for        one of the weld layers of the parts to be welded (i.e., one of        the layers at the weld interface), the method of the invention        can allow the formation of beads (or menisci) at the end of the        weld interface thereby limiting effects related to damage or        incipient cracks.

BRIEF DESCRIPTION OF THE FIGS.

FIG. 1 gives a schematic perspective view of an induction weldinginstallation according to the present invention.

FIG. 2 gives a schematic cross-sectional view of an induction weldinginstallation according to the present invention.

FIG. 3 gives an enlarged cross-sectional view of zone A in FIG. 2 .

FIG. 4 gives an enlarged cross-sectional view of zone B in FIG. 2 .

FIG. 5 gives a schematic view of two parts welded according to oneembodiment of the method of the invention.

FIG. 6 gives a schematic view of two parts welded according to oneembodiment of the method of the invention.

FIG. 7 gives a cross-sectional view of a “spring effect” insert in afirst compression state according to one embodiment of the method of theinvention.

FIG. 8 gives a cross-sectional view of the “spring effect” insert inFIG. 7 in a second compression state according to one embodiment of themethod of the invention.

FIG. 9 gives a cross-sectional view of a “spring effect” insert in afirst compression state according to one embodiment of the method of theinvention.

FIG. 10 gives a cross-sectional view of the “spring effect” insert inFIG. 9 in a second compression state according to one embodiment of themethod of the invention.

FIG. 11 gives a cross-sectional view of an insert and weld platesaccording to one embodiment of the method of the invention.

FIG. 12 gives a schematic perspective view of one embodiment of theinstallation of the invention, comprising two compacting rollerspositioned either side of the parts to be welded according to oneembodiment of the method of the invention.

FIG. 13 gives a schematic perspective view of rectilinear relativemovement of two inductors in relation to the welding direction.

FIG. 14 gives a schematic perspective view of sinusoidal relativemovement of an inductor in relation to the welding direction.

FIG. 15 gives a schematic perspective view of rectilinear relativemovement of four inductors in relation to the welding direction,allowing surface welding.

FIG. 16 gives a schematic perspective view of a “U-shaped” insertaccording to one embodiment of the welding method of the invention viaresistive effect.

FIG. 17 gives a schematic perspective view of an insert according to oneembodiment of the welding method of the invention via resistive effect.

FIG. 18 gives a schematic perspective view of an insert according to oneembodiment of the welding method of the invention via resistive effect.

DETAILED DESCRIPTION

A more detailed, nonlimiting description of the invention is now given.

By “rigid part” it is meant a part which is not deformed or onlyscarcely deformed under its own weight. The rigidity of the part can becharacterized by testing the deformation of a test specimen of the partto be welded. For this testing, a test specimen is prepared cut from aportion of the part to be tested and having the narrowest thickness (ifof variable thickness), said test specimen having a length of 12 cm, andwidth of 1 cm. Rigidity is assessed by placing and centering the testspecimen on two supports spaced 10 cm apart. Under standard conditionsof temperature and pressure, the test specimen exhibits maximumdeflection at its center of 1 cm, corresponding to relative deformationin relation to length of no more than 10%.

By “part to be welded” it is meant a part comprising a thermoplasticmaterial. The part can be a part of single block structure (monolayerpart) or a part of multilayer structure (multilayer part).

By “composite material”, it is meant a material comprising reinforcingfibres in a matrix of thermoplastic material. By “non-compositematerial”, it is meant a material devoid of reinforcing fibres.

The expressions “composite material” “composite layer”, “ply” andsemi-finished product” are used interchangeably. The semi-finishedproducts can be tapes in the form of a web of fibres in a resin matrix.Preferably, the orientation of the reinforcing fibres is essentiallyunidirectional in semi-finished products. The semi-finished products canalso be fibrous fabrics or mats of unidirectional reinforcing fibresalso known as Non-Crimp Fabrics (NCF) impregnated with polymers.Semi-finished products can also be products comprising thermoplasticpolymer not reinforced with continuous reinforcing fibres, whether ornot formulated with various fillers.

By “compacted part” it is meant a part composed of at least twosuperimposed layers, laminated together and compacted.

By “deposited part” it is meant a part composed of at least twosuperimposed layers laminated together, without compaction by means ofpressure-applying equipment of autoclave or press type.

By “welded product” it is meant a product comprising at least two partssuch as defined above, welded together according to the method of thepresent invention.

Unless otherwise stated, all percentages concerning indicated quantitiesare volume percentages.

The invention is not limited to induction welding, but also relates towelding methods comprising the insertion of a heat-providing insert. Theheat can be generated via induction, via resistive effect, viavibration, via friction, via ultrasound, via use of laser, via a streamof hot gas or via conduction from an external heat source; preferablyvia induction or via resistive effect; more preferably via induction oralternatively via resistive effect.

With reference to FIGS. 1 to 4 , the installation 1 is intended for theimplementation of an induction method for welding two rigid parts 2, 3each comprising a thermoplastic material and having respective surfacesto be welded 10, 11 and respective free surfaces 17, 18. However, theinvention is not limited to the welding of two parts and could beapplied to the welding of more than two parts, for example the weldingof one part with a first other part and a second other part e.g.juxtaposed.

In particular, the parts 2,3 can be rigid in that they are not deformedor are only scarcely deformed under their own weight. Their rigidity canbe characterized by testing the deformation of a test specimen of thepart to be welded. For this deformation test, a test specimen isprepared cut from a portion of the specimen to be tested and having thenarrowest thickness (if of variable thickness), said specimen having alength of 12 cm and width of 1 cm. Rigidity is assessed by placing andcentering the test specimen on two supports spaced 10 cm apart. Understandard conditions of temperature and pressure, the test specimen showsmaximum deflection at its center of no more than 1 cm, corresponding torelative deformation in relation to length of no more than 10%.

More particularly, the parts 2,3 are just as rigid under the heatconditions of the welding operation i.e., they are rigid before, duringand after welding.

The parts 2, 3, in relation to each other, may comprise compatibledifferent thermoplastic materials, or else one same thermoplasticmaterial. By “compatible thermoplastic materials” it is meant misciblethermoplastic materials i.e., polymers the mixture of which has a glasstransition temperature intermediate between those of the polymers.Examples of suitable thermoplastic materials for the invention arepolyamides, polysulfones, phenylene polysulfide (PPS), polyimides inparticular polyetherimides (PEI), polyaryletherketones (PAEK) inparticular polyetherketoneketones (PEKK) and polyetheretherketones(PEEK), polyethylene terephthalate, polyolefins such as polypropylene,chlorinated polymers such as polyvinyl chloride (PVC) and polyvinylidenefluoride (PVDF), acrylic or methacrylic polymers. The thermoplasticmaterial can be an amorphous, crystalline or semi-crystallinethermoplastic material.

The polyamides can particularly be a polyphthalamide (PPA), PA 11, PA12, PA 6, PA 1010, PA 66, PA 46 or a copolyamide.

It may also be a combination of several of the above materials.

Advantageously, the parts 2, 3 comprise PPS, PEI or a PAEK such as PEEKor PEKK as thermoplastic material.

The parts 2, 3 may comprise fillers (including reinforcing fibres)and/or functional additives.

Among functional additives, particular mention can be made of one ormore surfactants, UV stabilizers, heat stabilizers, biocidal agents,impact modifiers and/or expanding agents.

The fillers may comprise fibres or non-fibrous fillers. Non-fibrousfillers are mineral fillers in particular such as alumina, silica,calcium carbonate, titanium dioxide, glass beads, carbon black,graphite, graphene and carbon nanotubes.

Fibrous fillers can be so-called chopped fibres or continuousreinforcing fibres.

In particular, the parts 2, 3 can independently be in compositematerial, said composite material comprising reinforcing fibres in amatrix of the thermoplastic material. Reinforcing fibres particularlyallow rigidification of the parts.

The reinforcing fibres can particularly be glass fibres, quartz fibres,carbon fibres, graphite fibres, basalt fibres, silica fibres, metalfibres such as steel fibres, aluminum fibres or boron fibres, ceramicfibres such as silicon carbide or boron carbide fibres, natural plantfibres, synthetic organic fibres such as aramid fibres or fibres ofpoly(p-phenylene benzobisoxazole) better known as PBO, or PARK fibres,or mixtures of such fibres. Preferably, they are carbon fibres or glassfibres, and more particularly carbon fibres.

Examples of composite materials are: carbon fibres in a PEKK matrix,glass fibres in a PEKK matrix, carbon fibres in a polyamide matrix e.g.PA11, PA12, PA6 or PA1010, glass fibres in a 10 polyamide matrix e.g.PA11, P12, PA6 or PA1010, carbon fibres in a polypropylene matrix, glassfibres in a polypropylene matrix, carbon fibres in a polyethyleneterephthalate matrix, glass fibres in a polyethylene terephthalatematrix, carbon fibres in a PEEK matrix, glass fibres in a PEEK matrix,carbon fibres in a PEI matrix, glass fibres in a PEI matrix, carbonfibres in a PPS matrix, glass fibres in a PPS matrix.

The parts 2, 3 can independently comprise from 25 to 80 volume %,preferably 45 to 70 volume % of reinforcing fibres e.g., carbon fibresand/or glass fibres relative to the total volume of the part. Inparticular, the parts 2, 3 can independently comprise reinforcing fibresin an amount of 25 to 30 volume %, or 30 to 35 volume %, or 35 to 40volume %, or 40 to 45 20 volume %, or 45 to 50 volume %, or 50 to 55volume %, or 55 to 60 volume %, or 60 to 65 volume %, or 65 to 70 volume%, or 70 to 75 volume %, or 75 to 80 volume % relative to the totalvolume of the part. The dispersion of reinforcing fibres in sufficientvolume percentage allows rigidifying of the parts to be welded, or ofthe constituent layers thereof.

The parts 2, 3 may comprise an amount of matrix in thermoplasticmaterial ranging from 20 to 75 volume %, preferably 30 to 55 volume %relative to the total volume of the part. In some embodiments, the parts2, 3 comprise an amount of matrix in thermoplastic material of 20 to 25volume %, or 25 to 30 volume %, or 30 to 35 volume %, or 35 to 40 volume%, or 40 to 45 volume %, or 45 to 50 volume %, or 50 to 55 volume %, or55 to 60 volume %, or 60 to 65 volume %, or 65 to 70 volume %, or 70 to75 volume %, relative to the total volume of the part.

In some embodiments, the parts 2, 3 can independently be essentiallycomposed, or composed, of the thermoplastic material. The parts 2, 3 canindependently be composed of a material devoid of any reinforcingelement e.g. reinforcing fibres (in particular carbon fibres and glassfibres).

By “essentially composed of the thermoplastic material”, it is meantthat the part contains solely the thermoplastic material and optionallyone or more functional additives; in particular, the part may compriseat least 90 volume % of the thermoplastic material, preferably at least95%, or at least 98%, or at least 99%, e.g., approximately 100%.

The parts 2, 3 may independently be free of any electrically conductivematerial.

The parts 2, 3 may also independently comprise from 0 to 30 volume % offillers and/or functional additives such as described above.

The parts 2, 3 may independently be single block structures oralternatively multilayer structures.

When at least one of the parts 2, 3 is a multilayer structure, thelayers can be the same or differ from each other.

The above-mentioned characteristics in connection with the parts alsoapply to the layers individually.

Preferably, the part 2, 3 comprises (or consists of) several compositelayers (or “semi-finished products”) such as described above. The part2, 3 can be a compacted part or a deposited part.

The number of composite layers in the part 2, 3 can therefore vary from2 to 150, preferably from 4 to 40, more preferably from 6 to 30, ideallyfrom 7 to 25.

Aside from the parts 2,3, no other thermoplastic material is addedduring the welding method. In particular, no other thermoplasticmaterial is added at the interface of the surfaces to be welded 10,11,whether upstream or downstream of the insert 4. The weld joining betweenthe two welded parts 2,3 is therefore formed by the matrices ofthermoplastic material of the parts themselves, in particular viainterpenetration.

The weld product displays satisfactory mechanical performance onassembly. This mechanical performance on assembly can be evaluated forexample by measuring ultimate shear stress. Ultimate shear stress ismechanical stress applied to parallel to the surface of the weld productcausing destruction of the material at the weld interface. For example,in one known technique in accordance with standards prEN 6060 orISO4587, grooves perpendicular to the welding direction can be made oneach of the two surfaces of the weld product, thereby localizing shearforce thus generated at the weld interface. Shear strength correspondsto the force required to rupture the weld product divided by the arearesisting shear.

Preferably, the orientation of the reinforcing fibres is essentiallyunidirectional in each composite layer. More preferably, theunidirectional orientation of the reinforcing fibres differs from onelayer to another. Further preferably, two adjacent layers haveunidirectional orientations of the reinforcing fibres which essentiallyhave an angle of about 90° to each other; or which essentially have anangle of about 45° to each other. Alternatively, the reinforcing fibresin at least one of the composite layers, and in particular in each ofthe composite layers, can have several directions.

The thermoplastic material can be the same as or differ from one layerto another of a multilayer part 2, 3. Preferably, the thermoplasticmaterial is of same type (e.g., PEKK or PEEK or PPS) in all the layersof the part 2, 3. It may optionally comprise a different grade from onelayer to another, for example different viscosity, different molecularweight or different melting point. Alternatively, the grade of thethermoplastic material is the same in all the layers.

In some embodiments, when at least one of the parts 2, 3 is a multilayerstructure, the layer comprising the surface to be welded 10, 11 (in thepresent description also called “first layer”) comprises a thermoplasticmaterial having a lower melting point than the melting point of thethermoplastic material(s) of the other layers of the part 2, 3. Themelting point of the thermoplastic material of the first layer can be 10to 100° C. lower, preferably de 20 to 60° C. lower, more preferably 35to 50° C. lower than the melting point of the thermoplastic material(s)of the other layers of the part 2, 3.

The layer comprising the surface to be welded 10, 11 may also comprise athermoplastic material having lesser viscosity than that of thethermoplastic material(s) of the other layers of the part 2, 3.

As an example, for thermoplastic materials selected from amongpolyetherketoneketones (PEKK), the viscosity of the thermoplasticmaterial of the first layer can be 3 to 30 cm3/10 mn lower, preferably 5to 20 cm3/10 mn lower, more preferably 7 to 15 cm3/10 mn lower than theviscosity of the thermoplastic material(s) of the other layers of thepart 2, 3. The indicated viscosities are Melt Volume Index values (MVI)or Melt Volume Rate values (MVR) measured according to standardsISO/FDIS/1133_1 and ISO/FDIS/1133_2. Measurement is performed at 380° C.under a weight of 1 kg. The products are dried before MVI measurement.

The layer comprising the surface to be welded 10, 11 may also comprise alarger volume of thermoplastic material than the other layers of thepart 2, 3 or a smaller volume of reinforcing fibres than the otherlayers of the part 2, 3. The volume of thermoplastic material in thefirst layer can vary from 30 to 100%, preferably 45 to 80%, morepreferably 55 to 70%, relative to the total volume of said layer of thepart 2, 3. The layer comprising the surface to be welded, enriched withresin compared with the other layers of the part 2,3, is preferablyoriented at 0° to the welding direction.

For example, the parts 2, 3 in their outer portions may have strongfibre reinforcement whilst maintaining at the surfaces to be welded theamount of thermoplastic material required for good welding.

The presence of a layer comprising the surface to be welded 10, 11having lesser viscosity and/or a greater volume amount of thermoplasticmaterial provides for facilitated welding and/or better performancethereof and in particular can allow the forming of a bead (or meniscus)of thermoplastic plastic at the weld interface.

The layer comprising the surface to be welded 10, 11 may also comprise areinforcing material having strong cross density such as a woven carbonfabric.

The layer comprising the surface to be welded 10, 11 as thermoplasticmaterial, may also comprise a mixture of two or more thermoplasticspecies (e.g., a mixture of a polyetherimide and a PAEK), the otherlayers of the part 2, 3 as thermoplastic material only comprising asingle thermoplastic species.

The parts 2, 3 may independently be of constant thickness or of varyingthickness, for example varying in the welding direction D.

One advantage of the induction welding method of the invention is thatit is relatively little sensitive to the distance between the inductorand the surfaces to be welded 10, 11 of the parts 2, 3.

The installation 1 comprises a support to hold the parts 2, 3 to bewelded (not shown in FIG. 1 ). The support may also hold the parts 2, 3during welding operations, for example by clamping. This supportpreferably comprises a planar surface intended to hold the parts 2, 3but it may also be of any possible shape.

Advantageously, particularly in the case of heat-conducting materialssuch as a carbon reinforced composite, it can be useful to preheat thearea to be welded to a temperature which must always remain lower thanthe melting point of all the constituent materials of the structure tobe welded, using any suitable means. It is also possible to heat thesupport. For example, and in particular for PAEK, the temperature can be40 to 150° C. lower, preferably 50 to 120° C. lower, more preferably 70to 90° C. lower than the melting point. Pre-heating allows limiting ofthe temperature difference between the welded interface and theremainder of the parts 2,3, thereby limiting the flow of heat from theinterface towards the parts 2,3. Heating also provides better controlover crystallization of the materials, in particular in the weld area.Heating can be local, in the vicinity of, or perpendicular to the areasto be welded.

Advantageously, particularly in the case of heat-conducting materialssuch as a carbon reinforced composite, it may also be useful to maintainheating of the welded area at a temperature which must remain lower thanthe melting point of all the constituent materials of the structures tobe welded, using any suitable means e.g., infrared lamps or stream ofhot air. Maintained heating also allows better control over thetemperature of the interface to be welded. Heating also provides bettercontrol over crystallization of the materials in particular in thewelded area. Heating can be local, in the vicinity of, or perpendicularto the welded area.

The installation 1 comprises an insert 4. The insert 4 is a heatinginsert in that it is able to provide heat. The heat can be provided byany suitable means in particular via induction, via resistive effect,via vibration, via friction, via ultrasound, via use of laser, via astream of hot gas or via conduction from an external heat source.

For induction welding, the insert comprises a material comprising aninduction-sensitive material, and the heat of the insert is generatedthrough the generation of a magnetic field by at least one inductor 5.

By “induction-sensitive material”, it is meant a material capable ofbeing heated when subjected to a magnetic field, at least under certainconditions. In particular it may be a susceptor material or magneticfield absorber. Preferably, the insert 4 comprises a material that ismore induction-sensitive than the constituent materials of the parts 2,3 (carbon fibres in particular when applicable).

Preferably, the induction-sensitive material is a metalinduction-sensitive material. The metal material can be selected forexample from the group formed by iron, steel (e.g., stainless steel),aluminum, nickel-chromium, titanium, or a combination thereof.

The insert 4, as induction-sensitive material, may comprise or mayconsist of a ferromagnetic material having a Curie temperature Tc. Thisallows better control over the temperature to which the insert 4 isheated when implementing the welding method. If the temperature of theferromagnetic material is lower than Tc, this material will haveferromagnetic behavior and will be sensitive to induction. When thetemperature of the material reaches Curie temperature Tc, the materialwill become paramagnetic and the induction sensitivity thereof will bemodified; the temperature of said material can be maintained attemperature Tc.

The insert 4, particularly if it comprises a ferromagnetic material asinduction-sensitive material, can be fully or partially coated with afunctional coating providing anti-corrosion properties for example(anti-corrosion material) or facilitating the gliding of the insertbetween the two parts (material facilitating gliding of the insert 4between the parts 2, 3).

For welding using resistive effect, the insert 4 comprises anelectrically conductive material, and the heat of the insert isgenerated via resistive effect (or Joule effect). The resistive effectis generated by applying an electrical current. The insert canoptionally be fully or partially coated with an insulating material.

The shape of the insert 4 can be adapted to heating via resistiveeffect. A “U-shaped” insert 47 positioned at the end portion of two arms81 and 82 is illustrated in FIG. 16 . An insert 48 positioned at the endportion of two arms 83 and 84 is illustrated in FIG. 17 .

Preferably, the resistive conducting material can be selected from amongnickel alloys, lead alloys, titanium alloys, manganese alloys,nickel-chromium alloys, iron-chromium-aluminum alloys and nickel-copperalloys.

The insert 4 may comprise different zones comprising differentmaterials, to localize the heating zone at the surfaces to be welded.These materials can be assembled via brazing for example. Similarly, asillustrated in FIG. 18 , the insert may comprise a series of resistivezones 49 for example, mounted in parallel to homogenize temperaturealong the insert.

For welding using laser, the insert 4 can be heated directly by at leastone laser. Alternatively, the insert 4 may comprise a network of opticalfibres allowing the energy of laser heating to be directed towards thesurfaces to be welded.

For welding using a stream of hot gas, the insert 4 can be heateddirectly by the stream of hot gas, e.g., via contacting. Alternatively,the insert 4 may comprise at least one duct allowing circulation of thestream of hot gas inside the insert.

For welding using conduction, the insert 4 can be heated by any suitableexternal heat source.

The insert 4 is advantageously a plate. The insert 4 has a thickness of5 mm or less, preferable of 0.3 to 5 mm, more preferably of 0.3 to 3 mm,further preferably of 0.5 to 1.5 mm, still further preferably of 0.5 to1 mm. In some embodiments, the insert 4 has a thickness of 0.1 or less,or of 0.1 to 0.2 mm, or of 0.2 to 0.3 mm, or of 0.3 to 0.5 mm, or of 0.5to 1 mm, or of 1 to 1.5 mm, or of 1.5 to 2 mm, or of 2 to 2.5 mm, or of2.5 to 3 mm, or of 3 to 3.5 mm, or of 3.5 to 4 mm, or of 4 to 4.5 mm, orof 4.5 to 5 mm. By “thickness” it is meant the dimension between thesurfaces of the insert 4 in contact with the surfaces to be welded. Ifthe surfaces of the insert 4 are not planar and parallel to each other,the thickness corresponds to the maximum dimension between these twosurfaces. Such thicknesses ensure the rigidity of the insert, good heattransfer and scarce mechanical deformation of the rigid parts 2, 3 atthe time of inserting the insert 4 and makes welding of rigid partspossible. For effective welding between the two parts to be welded, thetemperature of the surfaces to be welded forming the weld interface mustbe higher than the melting point of the thermoplastic polymer whenpressure is applied to the area to be welded by the compacting roller(s)6. As illustrated in FIG. 11 , it is preferable to limit the distance“d” between the end of the insert 4 and the contact point of thesurfaces to be welded of parts 2, 3. This implies limiting the thicknessof the insert 4 to prevent stressing the parts 2,3 to be welded, or onethereof, beyond their elastic limit. Therefore, the thickness of theinsert 4 must be miniaturised, adapted and optimised accordingly, takinginto account the rigidity of the parts to be welded 2, 3, and musttypically have a thickness of 5 mm or less.

The insert 4 may have dimensions (e.g., length, width, thickness), shapeand/or properties (e.g., spring effect) adapted to the parts to bewelded and to the welding method (e.g., speed rate).

The insert 4 can have a width (perpendicular to the welding direction)at least equal to the width of the overlap area of the parts to bewelded 2, 3.

Alternatively, the insert 4 can have a width smaller than the width ofthe of the overlap area of the parts to be welded 2, 3, thereby forminga weld solely on part of the width of the overlap area. The insert 4 canbe of planar shape i.e., each of its two surfaces are planar. The planarsurfaces can be parallel to each other (zero angle). To optimize heatingof the surfaces to be welded in contact with the insert 4, the surfacesof the insert 4 can form a nonzero angle of bevel shape e.g., an inserthaving a planar bevel or an insert having a nonplanar bevel. Thesurfaces of the insert can have specific geometries adapted to theprofile of the parts 2, 3 to be welded. The insert can be of optimisedshape to promote heat transfer via contact with the substrates asillustrated in FIG. 11 . For example, the geometry of the susceptor canbe designed so that it is able to adapt to variable distances betweensubstrates.

In other embodiments, the insert 4 can have any other adapted shape, inparticular a nonplanar shape. The use of such insert of particulargeometry allows the welding of parts having non-planar surfaces to bewelded 10, 11. One example is illustrated in FIG. 6 . The insert 4 ispositioned at the end portion of an arm 8, and is preferably attached tothe arm 8.

An insert 41 “with spring effect” is illustrated in FIGS. 7 and 8 in twodifferent states of compression. Another insert 42 “with spring effect”is illustrated in FIGS. 9 and 10 in two different states of compression.It may happen that the surfaces to be welded have variable spacingbefore welding on account of the manufacturing tolerances of the parts2,3.

The induction welding method of the invention comprises insertion of theinsert 4 between the surfaces to be welded 10, 11 of the two parts 2, 3.At the time of welding, the insert 4 moves in relation to the parts tobe welded 2, 3, in the welding direction D.

This relative movement can be obtained by moving the parts to be welded2, 3, the insert 4 remaining fixed in relation to the support.Alternatively, and preferably, it can be obtained by moving the insert 4in relation to the support, the parts to be welded 2, 3 remaining fixedin relation to the support.

For example, the insert 4 can move in relation to the parts to be welded2, 3, in the welding direction D, at a rate of 50 to 1000 mm/min,preferably 100 to 500 mm/min.

At the time of relative movement of the insert 4 in relation to theparts to be welded 2, 3, in the welding direction D, the travel path ofthe insert (and of the parts to be welded) can be rectilinear.Alternatively, in particular when the width of the insert is smallerthan the width of the overlap area of the parts to be welded 2, 3, thetravel path of the insert may not be rectilinear. For example, theinsert may also move transversally, sinusoidally or incrementally, orotherwise.

For induction welding, the installation 1 also comprises at least oneinductor 5. When implementing the induction welding method of theinvention, the inductor 5 generates a magnetic field. The inductor hasoptimised geometry in relation the magnetic field to be applied: it mayor may not be composed of windings. If the geometry thereof compriseswindings, it can be an inductor having a single winding or severalwindings, these windings possibly being off-centered and/or oriented asa function of the materials to be welded.

In one embodiment, as illustrated in FIGS. 1, 2, 3, 13 and 15 , theinstallation may comprise a single inductor 5, 53. In anotherembodiment, as illustrated in FIG. 13 , the installation may comprise atleast two inductors 51 and 52, forming separate welding areas. Inanother embodiment, as illustrated in FIG. 15 , the installation maycomprise a series of at least two inductors e.g. a series of fourinductors 54, allowing surface welding of the two parts to be welded2,3.

The inductor 5 can be fixed in relation to the parts 2, 3. The inductor5 may therefore be of large size to allow heating of the insert 4 as itmoves; provision can also be made for a plurality of fixed inductors 5along the welding direction D. But preferably the inductor is mobile inrelation to the parts 2, 3. During the relative movement of the insert 4in relation to the parts to be welded 2,3, in the welding direction D,the travel path of the inductor can be rectilinear or it may benon-rectilinear. As illustrated in FIG. 13 , the inductors 51 and 52 mayhave a rectilinear travel path. As illustrated in FIG. 15 , the seriesof inductors 54 may also have a rectilinear travel path. As illustratedin FIG. 14 , the inductor 53 may have a sinusoidal travel path.

Advantageously, when implementing the welding method of the invention,the insert 4 and the inductor 5 move together in relation to the partsto be welded 2, 3 at the time of welding in the welding direction D.

By the expression “move together”, it is meant that they move at thesame time in the same direction (here the welding direction D) and atthe same speed.

Preferably, the arm 8 having the insert 4 at its end portion is attachedto the inductor 5.

The welding method of the invention may comprise a step to contact thesurfaces to be welded 10, 11 of the two parts to be welded 2, 3 byapplying pressure onto at least one of the two parts 2, 3 upstream of(i.e., in front of) and/or downstream of (i.e., behind) the position ofinsert 4 in relation to the welding direction D.

Therefore, the installation 1 may also comprise one or morepressure-applying elements. These pressure-applying elements can bepositioned behind and/or in front of the insert in relation to thewelding direction D. The pressure-applying elements allow theapplication of pressure on the parts 2, 3 so that they are pressed oneagainst the other. Preferably, when the installation 1 comprises severalpressure-applying elements, the pressures applied by each of thesepressure-applying elements are independent of each other.

For example, they may be one or more compacting rollers 6, positionedbehind the insert in relation to the welding direction D. The compactingroller(s) 6 promote interpenetration of the materials softened by thetemperature of the insert 4. They may also be several compacting rollers6 optionally surrounded by a continuous track of treads 16 to ensure acertain time of maintained compacting pressure. In these embodiments,each of the compacting rollers 6 can apply pressure and/or havekinematics that are coupled between rollers or else are independent ofeach other. The pressure-applying elements may comprise cooling means.They may also be independently subjected to vibration at an adaptedfrequency e.g., ultrasonic, to facilitate welding by promotinginterpenetration and macromolecular diffusion of the materials softenedby the temperature of the insert 4. The vibrations can be induced by avibrator 12. If the device comprises at least two compacting rollers,these may have the same or different diameters. These rollers may alsobe provided with continuous track. As illustrated in FIG. 12 (inductornot illustrated), two fixed compacting rollers 61 and 62 can bepositioned either side of the mobile parts 2,3, placed opposite eachother.

The pressure-applying elements may also consist of one or more pressurerollers positioned in front of the insert in relation to the weldingdirection D. This or these rollers ensure sufficient pressing togetherof the parts 2, 3. The positioning of the pressure-applying elements infront of the insert is particularly useful since the movement of theinsert between the parts 2,3 causes the two parts to draw away from eachother and may reduce the contact surfaces with the insert and reducewelding efficacy.

Preferably, the pressure-applying elements e.g., the compactingroller(s) 6, the compacting rollers surrounded by continuous track 16and/or the pressure roller(s) can move independently together withtravel of the insert 4, in relation to the parts to be welded 2, 3, atthe time of welding and in the welding direction D. In inductionwelding, they can be independently attached to the inductor 5. They maybe independently attached to the arm 8 which comprises the insert 4.

The welding method of the invention may comprise a step to cool thewelded parts by applying a heat-regulating block (not illustrated) ontoat least one of the two parts 2, 3 behind the position of the insert 4in relation to the welding direction D, or in front of thepressure-applying elements if any.

The heat-regulation block reduces the temperature of the free surfacesof the welded part relative to welding temperature, whilst maintainingthe surfaces to be welded and hence the weld interface at a temperaturehigher than the melting point of the thermoplastic polymer.

This cooling step provides control over the temperature gradient withinthe welded part and limits and even prevents decompaction.

The heat-regulating block is composed of a material having suitablethermal conductivity and can be temperature-regulated e.g., via thecirculation of a fluid. If the insert is heated by induction, theconstituent material of the heat-regulating block can advantageously beheat-conductive and electrically insulating.

The installation 1 may also comprise a controlled-temperature chamber14. This chamber 14 is preferably positioned behind the insert inrelation to the welding direction D. A chamber can also, oralternatively, be positioned in front of the insert in relation to thewelding direction D. Advantageously, at the time of welding it can movetogether with travel of the insert 4, in relation to the parts to bewelded 2, 3, in the welding direction D. In some embodiments, thechamber 14 is attached to the arm 8 comprising the insert 4. Ininduction welding, the chamber 14 is also or alternatively attached tothe arm 8 comprising the inductor 5. This controlled-temperature chamber14 allows one zone of the parts 2, 3 to be held at a specifictemperature e.g., to maintain one zone of the parts 2, 3 that has beensoftened by heating—e.g., by induction—at a recrystallizationtemperature to allow recrystallization under optimal conditions and toprevent post-cure of the parts after welding. It can also allow externalcooling of the parts outside the welding area (and in particular outsidethe chamber). In addition, when heating of the surfaces to be welded 10,11 is performed via convection, the presence of said chamber 14 canlimit perturbation of convection flows.

The controlled-temperature chamber 14 can be brought to the desiredtemperature by blowing a fluid inside the chamber 14, preferably hotair, by means of at least one blow tube 15.

The area located outside the temperature-controlled chamber 14 can bebrought to another temperature and for example can be cooled, inparticular by blowing a fluid preferably cold air by means of at leastone blow tube.

The controlled-temperature chamber 14 can be delimited by means of aflexible skirt e.g., in elastomer material. The flexible skirt can besecured for example to the periphery of an upper plate. With thisconfiguration it is possible to maintain an essentially closed chamberdespite any variations in height of the upper plate in relation to theparts 2, 3 and in particular it can adapt to parts 2, 3 of any shape.

The installation 1 may also comprise a second arm 9 having at its endportion a spacer element 7, optionally attached to the insert 4. Thespacer element 7 is inserted between the surfaces 10, 11 to be welded ofthe parts 2, 3. In particular, it provides limiting of friction betweenthe insert 4 and the parts 2, 3.

The spacer element 7 is preferably positioned in front of the insert 4in relation to the welding direction D. At the time of welding, it canadvantageously move together with travel of the insert 4, in relation tothe parts to be welded 2, 3, between the surfaces 10, 11 to be welded inthe welding direction D. The arm 9 comprising the spacer element 7 canbe attached to the arm 8 which comprises the insert 4. In inductionwelding, the arm 9 comprising the spacer element 7 can also oralternatively be attached to the arm 8 which comprises the inductor 5.

The spacer element can be of double convex shape (visible in FIG. 3 ),in that each of its surfaces are convex.

The spacer element can also be of mixed shape, in that one surface isconvex and the other is planar.

When the welding method of the invention is implemented, the insert 4can be in contact with each of the surfaces 10, 11 to be welded of thetwo parts 2, 3. Alternatively, it is possible that the insert may not bein contact with at least one of the surfaces 10, 11 to be welded of thetwo parts 2, 3, in particular it may not be in contact with any of thesurfaces 10, 11 to be welded of the two parts 2, 3.

Heating of the surfaces 10, 11 to be welded can therefore be obtainedvia conduction and/or via convection and/or via radiation from theinsert 4. The installation 1 may also comprise at least one pyrometer(not illustrated). When implementing the welding method of theinvention, the pyrometer continually or at point times measures thetemperature of the parts to be welded in the weld area. The pyrometer ispreferably positioned at the insert 4 in relation to the weldingdirection D.

The pyrometer is preferably positioned on one of the edges of the partsto be welded 2, 3 or alternatively a pyrometer is positioned on each ofthe edges of the parts to be welded 2, 3, in particular when the widthof the insert is at least equal to the width of the overlap area of theparts to be welded 2, 3. The method of the invention allows controlled,homogeneous warm-up of the parts to be welded 2, 3 in the weld area.Measurement of temperature at one of the edges, or at both edges, issufficient and allows extrapolating of the temperature over the entireweld area.

Alternatively, or additionally, a pyrometer can be positioned at thefree surface of one of the two parts, at any point of the weld area, inparticular if the width of the insert only represents a portion of thewidth of the overlap area of the parts to be welded 2, 3.

The pyrometer can also measure the temperature of the insert 4 on theedge of the parts to be welded.

Preferably, the insert 4 and the pyrometer, at the time of welding, movetogether in relation to the parts to be welded 2, 3, in the weldingdirection D. It can be attached to the arm 8 comprising the insert 4.

The installation may comprise a multi-weld device.

In one embodiment, the multi-weld device can allow the simultaneouswelding of at least three parts to be welded. Said device in particularmay comprise at least two inserts, positioned at the same level oroffset from the direction of welding. These multiple insertsrespectively allow the welding of the first part and the second part,the welding of the second part and the third part etc., and juxtaposingthereof. In another embodiment, the multi-weld device may allow thewelding of two parts at two separate points of the overlap area. Saiddevice may particularly comprise at least two inserts positioned at thesame level in relation to the welding direction and with certain spacingtherebetween. Each insert allows the welding of one portion of a firstpart and of a second part. After welding, only some portions of theoverlap area of the two parts will be welded together, the otherportions not being welded. In induction welding, the same result can beobtained by positioning a wide insert over the entire surface of theparts to be welded and by applying heat via several inductors providinglocalized heating and which travel above the part to be welded.

The welding method of the invention may comprise the formation of a bead(meniscus) 13 of thermoplastic material at the end of the weld interface(visible in FIG. 5 ). The formation of this bead (meniscus) is madepossible since the method of the invention allows heating of thesurfaces to be welded 10, 11 and hence softening of the thermoplasticmaterial of the parts 2, 3 as far as the end portion of these surfacesto be welded 10, 11 (in particular by adapting the dimensions of theinsert 4 to the surfaces to be welded 10, 11, the insert 4 thereforebeing able to extend as far as one or more ends of the surfaces to bewelded 10, 11, even to extend beyond one of more ends of the surfaces tobe welded 10, 11). The presence of a bead (meniscus) 13 at the end ofthe weld interface allows limiting of the risk of incipient crackformation. In an induction welding method based on heating the carbonfibres of the composite parts to be welded, the creation of currentloops close to the end portions of the weld interface is not possible,generally resulting in welds of lesser mechanical strength at thesepoints.

In some embodiments, the part 3 the furthest distant from the inductor 5is a multilayer structure having a conductive element as outer layer oras part of the outer layer (i.e., the layer the furthest distant fromthe surface 11 to be welded). The conductive element may particularly bea metal mesh e.g., in copper or bronze. The presence of this conductiveelement is particularly advantageous for the manufacture of fuselages inthe aeronautics sector, this element providing aircraft with protectionagainst lightening. The method of the invention has the advantage thatit is able to use localized heating at the interface between the twoparts to be welded. For example, for heating via induction, it ispossible to weld the parts 2, 3 without the magnetic field reaching theconductive element being sufficient to induce major heating of thiselement, preventing local degradation of the part through overheating ofthis conductive element. In addition, the method of the inventionprevents the conductive element from capturing a large part of theeffect of the magnetic field, which otherwise would result ininsufficient heating of the surfaces to be welded.

Movement of the insert 4 and/or inductor 5 (for induction welding)and/or of the pressure-applying elements and/or controlled-temperaturechamber 14 and/or the spacer element 7 can be obtained in automatedfashion via one or more robots, or else manually by an operator. Inparticular, for induction welding, the insert 4 and the inductor 5 canbe moved together by the same robot.

The weld temperature is a function of the temperature of the insert 4.For induction, the temperature of the insert 4 is itself dependent onthe power and frequency of the magnetic field delivered by the inductor5, on the travel speed of the insert 4 (e.g., together with the inductor5) and on the distance between the inductor 5 and the insert 4.

In some embodiments, the inductor 5 generates a magnetic field having afrequency of 10 Hz to 2 MHz, preferably 80 Hz to 300 kHz, morepreferably 100 Hz to 200 kHz.

In some embodiments of induction welding, the distance between theinductor 5 and the insert 4 remains constant throughout welding. Inother embodiments, this distance can vary, in particular if at least oneof the parts to be welded 2, 3 is of varying thickness.

In some embodiments, the method of the invention comprises a step tocontrol the temperature of the insert 4 (by means of a thermocouple orany other suitable temperature sensor) and a step for instant regulationof this temperature in particular by adjusting the travel speed of theinsert 4, and/or the power of the magnetic field delivered by theinductor 5, or the electrical power in the event of resistive heatingand/or any other relevant parameter e.g. via a conventional feedbackloop.

Thermal regulation of the insert has the advantage of allowing thesurfaces to be welded to be brought to a required temperature, adaptedfor assembly by welding. Several regulating modes can be applied.

For example, one regulating mode may obtain thermal regulation bymeasuring the temperature of the insert 4 with a device of laserpyrometer type. After previously quantifying the temperature differencebetween the control area accessible during welding (edge of insert 4, inthe vicinity of the parts 2,3) and the surfaces of the insert 4 incontact with the surfaces to be welded (inaccessible during the weldingphase), the power of the heating device such as the induction generatorfor heating via a magnetic field, or the current generator for resistiveheating, can be servo-controlled by the temperature of the insert 4 thatis accessible during the conventional welding phase.

Another regulating mode for the induction welding method can be the useof material having a Curie temperature to fabricate the insert 4. TheCurie point of a material significantly modifies the sensitivity thereofto a magnetic field and to induction phenomena. In this mode, twoconfigurations in particular can be used. One configuration is thecapacity of the material not to be heated by induction beyond thisparticular point (halting of phenomena of induced current and magneticdipoles). In this case, thermal regulation of the insert is physicallyensured as soon as the Curie point is crossed. However, if heating byinduction still occurs beyond this point (e.g., persistent inducedcurrent) the significant change in sensitivity to the magnetic field ofthe material at the Curie point (e.g., magnetic permeability) can bedetected by an adapted sensor and placed in the environment of thewelding device; this sensor provides indication that the Curie point hasbeen reached allowing electronic regulation of heating to be initiatedensuring servo-control of generator power. The parameter detected in theenvironment of the welding device can be the intensity of thesurrounding magnetic field (e.g., Hall effect sensor) affected by suddenchanges in the characteristics of the material of the insert 4 tomagnetic phenomena. Detection of a parameter of the environment couldalso entail variation in the impedance of the insert 4 coupled to theinductor.

Said embodiments are particularly advantageous for the assembly offuselage parts in the aeronautics sector, since control over weldingtemperature is required for qualification of an aeronautic process.

Another regulation mode for the welding method via resistive effect caninvolve measurement of the resistivity of the insert. Measurement ofresistivity, dependent on temperature, will allow servo control over thevalue of the current passing through the insert 4.

The method of the invention may also comprise a step to recordtemperature values of the insert 4 and/or travel speeds of the insert 4and/or the magnetic power delivered by the inductor 5, and/or thepressure applied to the parts to be welded 2, 3 and/or any otherparameter. This is particularly advantageous for the production offuselages in the aeronautics sector for which the recording of thesedata is required for qualification of an aeronautic process.

The welded parts 2, 3 of the invention can particularly be aircraftfuselage parts, such as fuselage skin parts, frames or stringers.

Alternatively, these parts can be aerospace or automotive parts, orparts for sports equipment.

The method of the invention can also be applied to the welding oftarpaulins whether or not structural, in particular in the field ofcivil engineering and water sports (mooring tarpaulins, boat sails . . .).

EXAMPLES

The following examples illustrate the invention but are nonlimiting.

In Examples 1 to 4, when producing at least one of the parts to bewelded using ATL technology (Automated Tape Lay-up), the first depositedlayer is a UD-Tape layer, the composition of which differs from that ofthe other constituent layers of the composite part.

Example 1

The composition of the first layer is 50 weight % PEKK Kepstan° 7002 and50 weight % carbon fibres. The composition of the other layers is 34weight % PEKK Kepstan° 7002 (marketed by Arkema) and 66 weight % carbonfibres.

In this example, the higher percentage of thermoplastic material in thefirst layer facilitates welding without changing the method forproducing the parts, and imparts this first layer with creep undercompacting pressure allowing a meniscus to be obtained at the endportion of the weld interface. In addition, said first layer cancompensate for some defects in the surfaces to be welded.

Example 2

The composition of the first layer is 34 weight % PEKK Kepstan° 6002(having a melting point of 303° C.) and 66 weight % carbon fibres. Thecomposition of the other layers is 34 weight % PEKK Kepstan® 7002(marketed by Arkema) (having a melting point of 333° C.) and 66 weight %carbon fibres.

The parameters of the method can be adapted so that the temperature ofthe insert allows melting of the thermoplastic material of the firstlayer without melting that of the other layers.

Alternatively, the constituent material of the insert comprisesferromagnetic material having a Curie point such that the “ceiling”temperature of the insert (and of its environment in the method)guarantees melting solely of the first layer.

Example 3

The composition of the first layer of the part is 50 weight % polyamideand 50 weight % glass fibres. The composition of the other layers is 34weight % polyamide and 66 weight % glass fibres.

Example 4

The composition of the first layer is 34 weight % of a first polyamidehaving a particular melting point and 66 weight % glass fibres. Thecomposition of the other layers is 34 weight % of a second polyamidehaving a higher melting point than the first polyamide, and 66 weight %glass fibres.

Example 5

In these first feasibility validation tests, a foil of thickness 0.8 mmwas used as insert. The substrates to be welded were composed of a PPSmatrix comprising a woven carbon fabric, and substrate thickness was 1.5mm. A ply of woven glass fabric in a PPS matrix was placed at the weldinterface to provide a non-conductive material at the interface.

Welding was performed according to the method of the invention. Weldingwas effective and it was observed that the assembly obtained iscohesive.

This example allowed verification that the welding of the invention isindeed obtained by means of heating the insert, and not solely onaccount of the presence of conductive fibres in the parts to be welded.

The weld interfaces were fully interpenetrated, and, after analysis ofthe weld joint, cohesive rupture of the thermoplastic matrix wasobserved.

Example 6—Weldability Via Induction of Non-Electrically ConductiveSubstrates

The parts to be welded were semi-finished products marketed under thetrade name Polystrand™ IE 7034B by PolyOne, corresponding to apolypropylene unidirectional thermoplastic tape. These semi-finishedproducts comprise 70 weight % glass fibres, have a thickness of 0.25mm/semi-finished product and gram weight of 354 g/m2. The thickness ofthe parts to be welded was 3 mm.

Two parts were welded using an industrial robot marketed by Kuka, and acurrent generator for inductor marketed by CEIA.

The welding parameters applied were the following:

-   Frequency: 200 kHz;-   Material of insert 4: Steel;-   Pmax limited to 20% of 12.5 kW;-   Distance between inductor and substrate: 5 mm;-   Welding temperature: about 200° C.,-   Travel speed of insert relative to the parts to be welded: 3.3    mms-1.

The shear stress (t) of the welded product was 11.8 MPa, namely 97% ofthe reference shear stress (part without weld) according to standardprEN 6060, the reference (100%) being performed according to the samestandard on 5 test specimens.

Example 7—Weldability by Induction of PEKK and Carbon Fibre Substrate

The parts to be welded were semi-finished products comprising PEKKhaving a melting point of 333° C. and carbon fibres. These materialsunderwent a consolidation step in an autoclave and lay-up as follows:

-   Orientation sequence (orientation in each fibre layer)+45°, 0°,    −45°, 90°, repeated 6 times with a plane of symmetry at the third    repeat;-   Volume percent of fibres (VPF) in substrates to be welded: 60%±2%;-   Volume percent of fibres (VPF) in added interface material: 35%; 150    μm.

The added interface material was a unidirectional web positioned on thesurface to be welded so that the fibres are oriented at 0° in relationto the welding direction. The thickness of the parts to be welded was4.4 mm.

Two parts were welded using an industrial robot marketed by Kuka, and acurrent generator for inductor marketed by CEIA.

The welding parameters applied were the following:

-   Frequency: 200 kHz;-   Material of the insert 4: Steel;-   Pmax limited to 30% of 12.5 kW;-   Distance between inductor and substrate: 5 mm;-   Welding temperature: 465° C.,-   Travel speed of insert in relation to the parts to be welded: 3.3    mms−1.

The shear stress (t) of the welded product was 35 MPa, corresponding toa shear stress close to that of the material of the parts 2, 3.

Example 8—Temperature Guiding (or Thermal Regulation Mode) in the WeldArea Via

Measurement and Regulation (Differing from the One Envisaged with theCurie Point Material of the Insert 4)

The temperature at the weld area was tested using a pyrometer ofreference SH15/SLE by CEIA.

At a first test, a rectilinear groove was made in the middle of the freesurface of one of the two parts to be welded, in the welding direction.A first pyrometer was placed in this groove and a second side pyrometerwas placed on the edge of the parts to be welded, in the continuation ofthe insert. The pyrometers were moved together with the insert. Thistest showed that the welding temperatures respectively measured by thefirst pyrometer placed in the groove and the second pyrometer arecoherent (constant difference).

At a second test, the pyrometer was placed solely on the edge of theparts to be welded, in the continuation of the insert. The pyrometer wasmoved together with the insert.

These tests show that the use of a single pyrometer placed on the edgeof the parts to be welded and at the insert is sufficient to control andguide the temperature of the weld area. They also showed the need for athermal regulation mode of the static susceptor (before setting therobot in operation) that is dynamic, independent and adapted.

Example 9—Measurement of the Temperature of the Insert

The materials and conditions were the same as in Example 7.

At the groove test, a temperature on the edge of insert was observed of490° C. for a temperature in the center of the groove of 465° C.

Welding operations were then performed (without grooves) with atemperature on the edge of the insert 4 of 490° C.

Example 10—Indifferent Distance Between the Substrate and Inductor

The parts to be welded were semi-finished products marketed under thetrade name Polystrand™ IE 7034B by PolyOne, corresponding to aunidirectional polypropylene-based thermoplastic tape. Thesesemi-finished products comprise 70 weight % glass fibres, have athickness of 0.25 mm/semi-finished product and gram weight of 354 g/m2.The thickness of the parts to be welded was 2 mm.

Two parts were welded (cf. the devices in Example 6).

The welding and control parameters applied were the following:

-   Frequency: 200 kHz;-   Material of the insert 4: Steel;-   Pmax limited to 25% of 12.5 kW;-   Distance between inductor and substrate: 5 mm, 10 mm and 15 mm;-   Welding temperature: about 180° C.

These tests show that it is possible to maintain a constant regulatedtemperature of the susceptor, irrespective of inductor/substratedistance. The effective power of the generator adapts and increases withthe distance under consideration.

These tests further show that, for the tested material, the temperatureof the outer surface (free upper surface) is 110 to 120° C. under steadyconditions, hence well below the melting point of the polypropylenematrix, irrespective of the distance between the inductor and substrate.

These tests were duplicated on woven carbon/PPS composites of thickness1.8 mm (reference CETEX TC1100 by Tencate): the conclusions were thesame with inductor/substrate distances of 10, 12 and 15 mm. Theregulated temperature was 300° C., surface temperature stabilized at245° C.

1. A method for welding at least two rigid parts comprising athermoplastic material and having respective surfaces to be welded,comprising: inserting an insert between the surfaces to be welded of thetwo rigid parts, said insert having a thickness of 1.5 mm or less;providing heat via said insert; wherein the insert moves in relation tothe two rigid parts to be welded at the time of welding, in a weldingdirection.
 2. The method according to claim 1, wherein the heat isprovided by said insert, the latter being heated via induction, viaresistive effect, via vibration, via friction, via ultrasound, via useof laser, via a stream of hot gas or via conduction from an externalheat source.
 3. The method according to claim 1, wherein the insertcomprises an induction-sensitive material, and the heat of the insert isprovided by the generation of a magnetic field by at least one inductor.4. The method according to claim 1, further comprising the contacting ofthe surfaces to be welded of the two rigid parts to be welded byapplying pressure onto at least one of the two rigid parts upstreamand/or downstream of the position of the insert in relation to thewelding direction.
 5. The method according to claim 1, wherein theinsert is in contact with each of the surfaces to be welded of the tworigid parts.
 6. The method according to claim 1, wherein at least one ofthe two rigid parts is made of composite material comprising reinforcingfibres in a matrix of thermoplastic material.
 7. The method according toclaim 1, wherein the thermoplastic material is selected from the groupof polyamides, polyimides, polyaryletherketones andpolyetheretherketones, polyethylene terephthalate, polyolefins,phenylene polysulfide, polysulfones, chlorinated polymers, acrylic ormethacrylic polymers.
 8. The method according to claim 1, wherein atleast one of the two rigid parts, are a multilayer structure.
 9. Themethod according to claim 1, wherein the two rigid parts are aircraftfuselage parts.
 10. An installation for welding at least two rigid partscomprising a thermoplastic material and having respective surfaces to bewelded comprising: a support to hold the two rigid parts to be welded;an arm comprising at its end portion a heating insert having a thicknessof 1.5 mm or less, configured to be inserted between the surfaces to bewelded of the two rigid parts; the insert being configured to move inrelation to the two rigid parts to be welded at the time of welding, ina welding direction.
 11. The installation according to claim 10, furthercomprising a heat-generating device of said insert via induction, viaresistive effect, via vibration, via friction, via ultrasound, vialaser, via a stream of hot gas or via conduction from an external heatsource.
 12. The installation according to claim 11, wherein theheat-generating device is at least one inductor, and said insertcomprises an induction-sensitive material.
 13. The installationaccording to claim 10, further comprising one or more compacting rollersand/or one or more pressure rollers.
 14. The installation according toclaim 13, wherein the compacting roller(s) are configured to besubjected to vibration at an adapted frequency.
 15. The installationaccording to claim 10, wherein the support is configured to be heated.16. The installation according to claim 10, comprising acontrolled-temperature chamber.
 17. The installation according to claim16, wherein the controlled-temperature chamber comprises a flexibleskirt.
 18. The method according to claim 1, wherein the insert has athickness of from 0.3 to 1.5 mm.
 19. The method according to claim 1,wherein the insert has a thickness of from 0.5 to 1.5 mm.
 20. The methodaccording to claim 1, wherein the insert has a thickness of from 0.1 to1 mm.
 21. The installation according to claim 10, wherein the insert hasa thickness of from 0.3 to 1.5 mm.
 22. The installation according toclaim 10, wherein the insert has a thickness of from 0.5 to 1.5 mm. 23.The installation according to claim 10, wherein the insert has athickness of from 0.1 to 1 mm.